Substrata of photographic films and papers are usually coated with aqueous solutions of hydrophilic colloid materials in an uninterrupted coating process. To this end supply rolls Or webs of material to be coated are joined together (spliced) prior to the coating process. This is usually accomplished by means of a piece of tape 6, which is glued to a trailing edge 3 of a preceding web 1 and a leading edge 4 of a newly supplied web 2 by means of an adhesive 5, as shown in FIG. 1. The resulting continuous web can then be uninterruptedly coated with at least one coating layer 8. However, when said coating layer 8 traverses the trailing edge of the splicing tape 6, the coating layer 8 cannot instantaneously make contact with the surface of the succeeding web 2. As a consequence air 7, entrained at the trailing edge of said splicing tape 6, may be entrapped between the coating layer 8 and the surface of the succeeding web 2. This causes several problems.
Firstly, the entrapped air may cause bubble defects, as shown in FIG. 2. These bubble defects result in areas of non-uniformity in the coating distribution which makes the product unsuitable for sale, Furthermore, at the areas of non-uniformity the coating layer can be locally thicker, requiring more time to dry. Consequently, when said bubbled areas reach a first pass roll that guides the coated web through the process, the liquid coating layer may not have completely dried. When passing said first pass roll the bubbles may burst and the not yet completely dried coating may contaminate the first roll and subsequent pass rolls. The contamination will hit the passing web various times before being completely removed by the passing web. This results in a considerable loss of coated products and in delays in the production, due to necessary cleaning of the coating apparatus.
As the coating speed is increased, the above-signalised problems will increase accordingly, since more air will become entrapped between the coating layer and the web surface behind the trailing edge of the splicing tape.
Various methods and countermeasures have been suggested to prevent the above described coating non-uniformities and coating defect problems.
Bourns et al. (Eastman Kodak, DE 1,805,734) suggests in 1969 to use filler material to smooth the gap between the web, the coating layer and the trailing edged of the splicing tape. In addition a special, tapered splicing tape is disclosed to reduce said gap and the unevenness and bubbles.
A different method is disclosed in 1979 by Heetderks et al. (Eastman Kodak, U.S. Pat. No. 4,172,001). The method involves the use of two different splicing tapes, a first tape being sufficiently strong for constituting the actual connection between the two webs and a second, very thin tape, placed on top of the trailing edge of the first tape, for covering the gap between the web, the coating layer and the trailing edge of the first splicing tape.
In 1977 Takagi et al. (Fuji Photo Film, U.S. Pat. No. 4,024,302) discovers that a small area of coarsened web surface immediately following the trailing edge of the splicing tape can prevent unevenness and coating discontinuities. A similar method is proposed by Deprez in 2001. (Eastman Kodak, U.S. Pat. No. 6,197,148) involving the application of a rough layer immediately after the trailing edge of the splicing tape.
DeRoeck et al. (Agfa Gevaert, U.S. Pat. No. 4,235,655) discloses yet another method in 1980. The splicing tape used in this application has its leading edge adhered to an upper surface of the trailing end of a preceding web and its trailing edge adhered to a lower surface of the leading end of a succeeding web. The webs are separated from each other over a distance of at least ten times their thickness. The tape thus forms a flexible connection between the two webs, wherein both discontinuities at the upper side of the webs to be coated are “step-up” discontinuities, which are known to be less harmful than “step-down” discontinuities.
A completely different method is developed by Verkinderen at al. (Agfa-Gevaert, U.S. Pat. No. 4,269,647) In this method a splicing tape is adhered against a lower side of the webs, after which a fast-drying hydrophobic coating is applied to the upper side around the splicing tape area. Subsequently the upper surface of the webs and tape is coated with a hydrophilic emulsion coating.
Finnicum (Eastman Kodak, U.S. Pat. No. 5,154,951) developed in 1992 a method and an apparatus to reduce the coating defects caused by the splicing tape by controlling a vacuum at the upper side of the web when the splice arrives at the coating apparatus, in such a way, that no air gets entrapped between the coating layer and the trailing edge of the splicing tape. This prevents the formation of coating defects.
With the known methods the splice induced coating defects may be reduced in some cases, but the splices remain a Source of potential problems. The more so as the coating speed has increased since the disclosure of the above-cited prior art. Furthermore, all above cited prior-art countermeasures require modifications to the splicing process and the need for additional operations and means, making the splicing process more complex. In fact, some of the intended improvements, such as the suggested changes to the shape of the splicing tape and the tape material may even bring on new problems.
The object of the invention is to provide an improved method for splicing successive webs to one continuous web, wherein the occurrence of non-uniformities and coating defects during subsequent coating of the spliced web is reduced, preferably eliminated.